Around 1 in 1,000 Israelis who caught COVID has been reinfected, a large-scale study has shown, indicating that repeat infection is more common than widely thought.
Maccabi Healthcare Services found that of 149,735 members who were coronavirus-positive between March 2020 and January 2021, 154 had been reinfected, according to a new research paper that offers “real-world data of SARS-CoV-2 reinfection in a large-scale population cohort.”
During this time frame Israel was not vaccinating recovered patients, so the results are an indicator of the effectiveness of natural immunity arising from infection.
The study, believed to be the largest of its kind, covers almost a fifth of Israelis who have been infected, and if it accurately represents the country as a whole, suggests there have been around 800 reinfections in Israel.
The researchers, from Maccabi and Ben Gurion University, only counted patients who had at least 100 days between first being diagnosed with coronavirus and then testing positive again, to eliminate the possibility that the initial infection was lingering. One in five reinfected people had more than 200 days between their first and second infection.
“Reinfection proportion, albeit small, is not insignificant,” they wrote in their paper, which has been posted online but not yet peer-reviewed. “As time passes the potential for reinfection increases.”
Prof. Cyrille Cohen, head of the immunotherapy laboratory at Bar Ilan University, who was not involved in the study, said it generated an “important finding.” He said: “There is nothing such as ‘100 percent’ in biology, so there are reinfections, and these data reveal the frequency of these occurrences.”
The new data points to an ominous pattern that suggests that reinfection may be a growing problem for much of the world that isn’t yet vaccinated.
Some 99 of the 154 reinfections were detected in January, the month when the British variant became widespread in Israel. The researchers said that the rise in variants may account for the reinfection spike. Given that further variants are feared, this could suggest that reinfections will increase.
Cohen believes this is a real concern, and pointed out that a variant that emerged in Brazil called P.1 has been found more likely to reinfect recovered patients than earlier versions of the coronavirus.
There have been numerous reports of reinfection worldwide, but few cases have been documented or had details released in the public domain. An online tracker currently lists just 66 such cases globally. But this is viewed as an underestimation, as scientists say that most reinfected people will never end up getting tested because they will be asymptomatic or have very mild symptoms, thanks to their immune system largely suppressing the virus.
Israel’s highly digitalized medical record-keeping means that healthcare providers can easily compute information on patterns in medical records, like reinfection, hence the new Maccabi data.
Some scholars met the data with caution, however. Prof. Galia Rahav, head of infectious diseases at Sheba Medical Center, said that despite the long interval between tests in the study, she suspects some swabs picked up lingering infections.
“I’m a little afraid to say it’s one in a thousand and I don’t think it’s true — I think some of it is remnants,” said Rahav, who has treated two patients with reinfections confirmed by sequencing, and thinks such occurrences are rarer than the new research suggests. In Rahav’s view, the study would only have full credence if it sequenced each case and found beyond doubt that it was a new infection.
The new study puts the risk of infection for previously infected people at a similar level to that of vaccinated people. This is more common than popularly believed — but less frequent than suggested by a recent Danish study.
Published in the Lancet earlier this month, that study examined results from 11,068 people who tested positive during Denmark’s first wave and found that 72 of them — 0.65% — tested positive again in the second wave.
The Danish study deployed a control group and concluded that protection arising from infection reduced the chance of a new infection by 80%. The Israeli study did not include a control group so doesn’t have parallel statistics.
The Israeli data was also more reassuring for the elderly than the Danish data was. In Denmark, the 80% protection fell to 47% among those 65-plus. In Israel, the 10-19 age group had the highest number of reinfections, followed by 20-29 and 30-39. Older age groups had smaller numbers of reinfections. There is no clear explanation for the differences, and further studies are expected.
While the Israeli authors acknowledged that reinfection is rare, they stressed that their data shows it is real, and suggested that authorities take it more seriously, challenging the widespread assumption that recovered people are safe from reinfection. They wrote: “Health policymakers should acknowledge the possibility of reinfection and reconsider the differential message to the recovered population.”